An iron and steel structure represented by a bridge, for example, is formed in such a manner that a cutting member obtained by cutting a steel plate in a desired shape is welded to form a great number of structural members, and these structural members are assembled. Therefore, the end portion of the cutting member, which constitutes the individual structural member, is classified into a welded surface that is welded to the other cutting member and an exposed surface that is exposed without being welded.
The cutting member of the structural member constituting the iron and steel structure is cut by employing a gas cutting method or a plasma cutting method. In this case, a bevel cutting by which an angle according to the thickness of the cutting member is provided is performed to the portion that is to become the welded surface, while a vertical cutting by which the cutting plane is vertical to the surface of the steel plate is performed to the exposed surface. When the target cutting member is welded to the member to which a rib has already been welded and has a function of avoiding the rib, in particular, the bevel cutting and the vertical cutting are continuously performed in one cutting process (one cutting line).
On the other hand, since almost all parts of the structural member are exposed to air in the iron and steel structure, the surface is coated so as to prevent the generation of rust. In this case, the exposed surface is also coated. However, the exposed surface is formed by vertically cutting the steel plate, so that the intersection (edge) of the cutting plane constituting the exposed surface and the front and backside surfaces of the steel plate is substantially at right angles. Accordingly, there arises a problem that the coated film is easy to be separated from this edge.
In order to prevent the coated film from being separated from the edge between the exposed surface and the front and backside surfaces, the edge is formed to have a dull angle by the execution of secondary fabrication by means of a sander or a chamfering machine along the exposed surface, and then, the resultant is coated.